Experimental deposition of Alzheimer amyloid beta-protein in canine leptomeningeal vessels

Neurobiology of the aging dog

Aged canines naturally accumulate several types of neuropathology that may have links to cognitive decline. On a gross level, significant cortical atrophy occurs with age along with an increase in ventricular volume based on magnetic resonance imaging studies. Microscopically, there is evidence of select neuron loss and reduced neurogenesis in the hippocampus of aged dogs, an area critical for intact learning and memory. The cause of neuronal loss and dysfunction may be related to the progressive accumulation of toxic proteins, oxidative damage, cerebrovascular pathology, and changes in gene expression. For example, aged dogs naturally accumulate human-type beta-amyloid peptide, a protein critically involved with the development of Alzheimer's disease in humans. Further, oxidative damage to proteins, DNA/RNA and lipids occurs with age in dogs. Although less well explored in the aged canine brain, neuron loss, and cerebrovascular pathology observed with age are similar to human brain aging and may also be linked to cognitive decline. Interestingly, the prefrontal cortex appears to be particularly vulnerable early in the aging process in dogs and this may be reflected in dysfunction in specific cognitive domains with age.

Immunohistochemical investigation of the brain of aged dogs. I. Detection of neurofibrillary tangles and of 4-hydroxynonenal protein, an oxidative damage product, in senile plaques

In the aging dog brain lesions develop spontaneously. They share some morphological characteristics with those of Alzheimer 's disease in man. Diffuse and primitive plaques are well known, whereas neuritic plaques rarely develop. Neurofibrillary tangles have not been seen in the canine. The aim of the present investigation was to study major age-related changes of the dog's brain using paraffin sections with respect to cross-immunoreactivity of tau, A beta protein and other immunoreactive components including hydroxynonenal protein, which is a marker for oxidative damage. The occurrence of neurofibrillary tangles and of the protein tau therein was studied in serial brain sections of two dogs with the Gallyas stain and by immunohistochemistry with three different antibodies against tau. Senile plaques were stained with a monoclonal anti-A beta (residues 8-17), polyclonal anti-apolipoprotein E and a monoclonal antibody against 4-hydroxynonenal (HNE). Amyloid deposits and controls were screened by Congo red staining viewed in fluorescent light, followed by polarized light for green birefringence. With the Gallyas stain and one of the antisera against tau, neurofibrillary tangles were revealed in a similar dispersed pattern, whereas the other antitau antisera gave negative results. With the anti-HNE a positive reaction was found in cerebral amyloid deposits and in vascular wall areas where amyloid deposition was confirmed by Congo-red staining, and in perivascular cells and in some neurons. These results indicate that the canine with his tangles and plaques which show oxidative changes, forms a spontaneous modelfor understanding the early changes and their interrelationships in Alzheimer's disease.

Prominent cerebral amyloid angiopathy in transgenic mice overexpressing the london mutant of human APP in neurons

Deposition of amyloid beta-peptide (Abeta) in cerebral vessel walls (cerebral amyloid angiopathy, CAA) is very frequent in Alzheimer's disease and occurs also as a sporadic disorder. Here, we describe significant CAA in addition to amyloid plaques, in aging APP/Ld transgenic mice overexpressing the London mutant of human amyloid precursor protein (APP) exclusively in neurons. The number of amyloid-bearing vessels increased with age, from approximately 10 to >50 per coronal brain section in APP/Ld transgenic mice, aged 13 to 24 months. Vascular amyloid was preferentially deposited in arterioles and ranged from small focal to large circumferential depositions. Ultrastructural analysis allowed us to identify specific features contributing to weakening of the vessel wall and aneurysm formation, ie, disruption of the external elastic lamina, thinning of the internal elastic lamina, interruption of the smooth muscle layer, and loss of smooth muscle cells. Biochemically, the much lower Abeta42:Abeta40 ratio evident in vascular relative to plaque amyloid, demonstrated that in blood vessel walls Abeta40 was the more abundant amyloid peptide. The exclusive neuronal origin of transgenic APP, the high levels of Abeta in cerebrospinal fluid compared to plasma, and the specific neuroanatomical localization of vascular amyloid strongly suggest specific drainage pathways, rather than local production or blood uptake of Abeta as the primary mechanism underlying CAA. The demonstration in APP/Ld mice of rare vascular amyloid deposits that immunostained only for Abeta42, suggests that, similar to senile plaque formation, Abeta42 may be the first amyloid to be deposited in the vessel walls and that it entraps the more soluble Abeta40. Its ability to diffuse for larger distances along perivascular drainage pathways would also explain the abundance of Abeta40 in vascular amyloid. Consistent with this hypothesis, incorporation of mutant presenilin-1 in APP/Ld mice, which resulted in selectively higher levels of Abeta42, caused an increase in CAA and senile plaques. This mouse model will be useful in further elucidating the pathogenesis of CAA and Alzheimer's disease, and will allow testing of diagnostic and therapeutic strategies.

Apolipoprotein E, smooth muscle cells and the pathogenesis of cerebral amyloid angiopathy: the potential role of impaired cerebrovascular A beta clearance

Cerebral amyloid angiopathy (CAA) is caused by the deposition of beta-amyloid (A beta) in Alzheimer disease brains. It also occurs isolated, representing a major cause for cerebral hemorrhage in the elderly. The E4 genotype of apolipoprotein E (ApoE) is a risk factor for CAA; however, the molecular mechanism underlying this genetic association is unknown. Various findings suggest that cerebrovascular A beta is derived from the soluble A beta contained in the cortical extracellular space or the cerebrospinal fluid (CSF) that communicates and surrounds small cortical or leptomeningeal vessels. CAA deposits are always intimately associated with smooth muscle cells (SMCs) or SMC-derived pericytes. As we have previously reported, SMCs internalize A beta in vitro via a lipoprotein pathway involving ApoE and the low-density lipoprotein receptor family. Internalized A beta is subsequently located to lysosomes, suggesting its intracellular degradation. We show that A beta is internalized via multiple pathways, because class A and class B scavenger receptors are also colocalized to A beta-containing endosomes in SMCs, and A beta uptake is inhibited by various scavenger receptor antagonists. It has been recently shown for different cell types that the cellular uptake of ApoE is more efficient for the ApoE3 isoform when compared to ApoE4 and that this isoform-specific difference depends on the presence of heparan sulfate proteoglycan (HSPG). HSPG is produced by SMCs and promotes A beta fibrillogenesis. We propose a pathogenetic model of CAA, in which the ApoE- and HSPG-mediated clearance of CSF-derived A beta peptides by SMCs protects the vascular extracellular matrix against critical A beta concentrations. Impairment of this pathway or its reduced efficiency in carriers of the ApoE4 genotype may increase the risk of developing CAA.

Animal models of cerebral beta-amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is a significant risk factor for hemorrhagic stroke in the elderly, and occurs as a sporadic disorder, as a frequent component of Alzheimer's disease, and in several rare, hereditary conditions. The most common type of amyloid found in the vasculature of the brain is beta-amyloid (A beta), the same peptide that occurs in senile plaques. A paucity of animal models has hindered the experimental analysis of CAA. Several transgenic mouse models of cerebral beta-amyloidosis have now been reported, but only one appears to develop significant cerebrovascular amyloid. However, well-characterized models of naturally occurring CAA, particularly aged dogs and non-human primates, have contributed unique insights into the biology of vascular amyloid in recent years. Some non-human primate species have a predilection for developing CAA; the squirrel monkey (Saimiri sciureus), for example, is particularly likely to manifest beta-amyloid deposition in the cerebral blood vessels with age, whereas the rhesus monkey (Macaca mulatta) develops more abundant parenchymal amyloid. These animals have been used to test in vivo beta-amyloid labeling strategies with monoclonal antibodies and radiolabeled A beta. Species-differences in the predominant site of A beta deposition also can be exploited to evaluate factors that direct amyloid selectively to a particular tissue compartment of the brain. For example, the cysteine protease inhibitor, cystatin C, in squirrel monkeys has an amino acid substitution that is similar to the mutant substitution found in some humans with a hereditary form of cystatin C amyloid angiopathy, possibly explaining the predisposition of squirrel monkeys to CAA. The existing animal models have shown considerable utility in deciphering the pathobiology of CAA, and in testing strategies that could be used to diagnose and treat this disorder in humans.

Canine leptomeningeal organ culture: a new experimental model for cerebrovascular beta-amyloidosis

Cerebral amyloid angiopathy (CAA) is a neuropathological feature of Alzheimer's disease and a common cause of cerebral hemorrhage in the elderly. The pathogenetic mechanisms leading to the deposition of Alzheimer amyloid beta-protein (A beta) in cortical and leptomeningeal vessel walls are unknown. There are no experimental models which reproduce the pathological changes of CAA. In this study, leptomeninges from young and old dogs with pre-existing CAA were cultured in cell culture medium or cerebrospinal fluid and their viability, histological appearance and metabolic activity were analyzed during the culture. In addition, living leptomeninges of old and young dogs were incubated with fluorescein-conjugated A beta and the uptake of A beta was studied by fluorescence microscopy. Leptomeninges from young and old dogs were viable up to 8 weeks in culture. They contain many small- and medium-sized arterioles, the main vessel type affected by CAA. Histology and immunohistochemistry showed excellent preservation of the vessel wall microarchitecture up to 4 weeks in culture. The cultures were metabolically active as shown by the de novo production of beta-amyloid precursor protein. Exogenously added A beta was focally deposited in the vessel walls of old, but not young dogs. In conclusion, the organ culture of canine leptomeninges is easy to perform and appears suitable to investigate the pathogenesis and the progression of CAA.